NON-ISOCYANATE CURING FOAM SYSTEM

An innovative, non-isocyanate curing PU foam System.

Current technologies for polyurethane foams rely on the reaction of a polyol and an isocyanate in the presence of a suitable catalyst, a blowing agent and additives such as a flame retardant, surfactants, pigments, dyes, fillers, or other components. Foam properties are modified by type and concentration of reactants and additives in formulations delivering different densities, rigidities, strength and structure. One-component-foams (OCF) have all formulation ingredients in a single (aerosol) can which after spraying gives a froth with cellular structure that cures by air humidity. Typically, substrates to build OCF-formulations are aromatic polyisocyanates,like polymeric MDI ( “pMDI”, well known in the construction foam industry.

Inherent to OCF isocyanate-curing is the excess of pMDI to the polyol counterpart, to guarantee sufficient free terminal isocyanate groups in the prepolymer. Water (e.g., from humidity) is thus allowed to react with formation of a free amine and CO2 accounting for post-expansion. Unfortunately, significant residues of unreacted isocyanate monomers like MDI in final formulation are a growing safety concern for end users. Upcoming changes in governmental regulations imply adjusted safety labelling, disposal as hazardous waste, and a lowering of the free monomeric content below 1% w/w, all of which are key drivers for industrial manufacturers to develop isocyanate free PU foam products.

To anticipate on these radical changes, the development of an alternative and self-curing foam system which is cured by oxygen from air was pursued. The concept is based on implementing chemistry with smaller ecological footprint and, most importantly, aims at providing a safer solution for PU foams to the user. Inspiration was found in the use of urethane (meth)acrylate prepolymers which currently find applications in adhesives, electronics, coatings, and flooring among others. Indeed, such materials tend to cross-link with formation of PU networks, particularly when radical initiation occurs photochemically or by electron beam. Obviously, this approach is less useful for PU foam hence a more suitable curing system needed to be designed. Radical initiators active at ambient temperatures, were found to successfully cure the urethane prepolymers. These compounds, alkylboranes, have very low activation energies for reaction with oxygen, giving instant decomposition and radical formation upon exposure to air. The behavior of these species was fully elaborated by EPR spectroscopy, providing useful mechanistic insights in the underlying chemistry. As such, it was possible to tune reactivity of the curing system to match with selected prepolymers and formulations thereof. Indeed, initial trials showed too fast of a curing on the interphase between froth and atmosphere, where obviously oxygen concentration is highest. As a result, premature skin formation occurred that interfered with a still expanding propellant eventually rupturing the foam structure. Deeper understanding of radical kinetics allowed to mitigate this effect, but several re-iterations on the formula composition were needed for example to tackle incompatibility of traditional PU additives with the newly-designed curing system. Finally, a proof-of-concept level was reached when a urethane acrylate formulation was prepared that fully cured within 2 minutes after release from the aerosol can. The curing mechanism as investigated was independent of temperature, no post expansion was observed while foam properties were good with uniform cellular structure and structural rigidity. This significantly faster curing, tuneable and predictable, now allows to decouple curing efficiency and final foam performance from variables like temperature and air humidity. Moreover, the applied technology is non-toxic, performing and provides perspective for a non-isocyanate future. The technology was patented, with several more applications pending.

Major advantage of the new process is the non-exposure of free isocyanate to DYI and construction workers, when applying these newly developed products.

The traditional one can PU OCF formulations contain more polymeric MDI as these zero-NCO formulations, which are more Urethanes Acrylates.

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